1. Field of the Invention
Embodiments of the present invention relate to a method for producing an electrophotographic photosensitive member.
2. Description of the Related Art
An electrophotographic photosensitive member including an undercoat layer and a photosensitive layer stacked, in that order, on a support has often been used as an electrophotographic photosensitive member used in an electrophotographic apparatus. As the photosensitive layer, a function separation type (laminated) photosensitive layer having a functionally separated structure that includes a charge generation layer containing a charge generation material and a charge transport layer (for example, a hole transport layer) containing a charge transport material (for example, a hole transport material) has often been used.
The undercoat layer is disposed in order to, for example, suppress charge injection, e.g., hole injection, from the support to the photosensitive layer. The undercoat layer having the function of blocking charge injection inhibits the occurrence of an image failure (a black spot).
For example, metal oxide particles are often incorporated into the undercoat layer in order to inhibit the accumulation of charges, e.g., electrons, in the undercoat layer. The inhibition of charge accumulation in the undercoat layer results in the inhibition of variations in light area potential due to the repeated use of the electrophotographic photosensitive member.
In particular, zinc oxide particles may be used as the metal oxide particles for use in the undercoat layer from the viewpoint of achieving good electrical properties, such as volume resistivity and a dielectric constant. Japanese Patent Laid-Open No. 2006-30700 discloses a technique for incorporating zinc oxide particles into an undercoat layer of an electrophotographic photosensitive member.
Each of the layers of the electrophotographic photosensitive member is commonly formed by forming a coating film of a coating liquid for the corresponding layer and drying and/or curing the coating film, the coating liquid being prepared by mixing a charge generation material, a charge transport material, metal oxide particles, or the like with a solvent together with a binding resin.
Thus, the binding resin used for the undercoat layer is required to have resistance to a solvent (solvent resistance, i.e., insolubility) contained in a coating liquid (a photosensitive layer coating liquid (a charge generation layer coating liquid or a charge transport layer coating liquid) or the like)) applied on or above the undercoat layer. The undercoat layer is required to have endurance in the repeated use of the electrophotographic photosensitive member and resistance to various environments, such as a high-temperature and high-humidity environment and a low-temperature and low-humidity environment. A curable resin may be used as the binding resin used for the undercoat layer from these points of view. In the case where the curable resin is used as the binding resin for the undercoat layer, the curable resin is required to be curable at a low temperature (to form a cured resin at a low temperature) from the viewpoint of achieving good productivity of the electrophotographic photosensitive member.
With respect to a resin that meets the requirement, there is a technique in which a resin curable at a low temperature is used as an undercoat layer. Japanese Patent Laid-Open No. 2004-198734 discloses a technique in which a curable resin (a urethane resin) prepared by allowing a blocked isocyanate having an isocyanate group blocked with a diethyl malonate structure to react with a polyol resin is used as a binding resin for an undercoat layer. Regarding an isocyanate compound, an isocyanate group may be blocked with a blocking agent in order to control the reactivity. An isocyanate compound with a blocked isocyanate group is referred to as a “blocked isocyanate compound”.
The results of studies by the inventors, however, revealed the following problems: That is, a coating liquid containing a polyol resin and a blocked isocyanate whose isocyanate group is blocked with a structure, such as a diethyl malonate structure, represented by the formula (1) increases in viscosity with time, in some cases. The reason for this is presumably that these materials are easily subjected to a curing reaction and thus the curing reaction proceeds gradually even in a normal temperature environment,
wherein, in the formula (1), X represents a single bond or an oxygen atom, and R1 and R2 each independently represent an alkyl group having 1 to 4 carbon atoms. In the case where in the formula (1), X represents an oxygen atom and where each of R1 and R2 represents an ethyl group, the formula (1) represents a diethyl malonate structure.
An increase in the viscosity of the coating liquid for an undercoat layer reduces the uniformity of a coat of the coating liquid for an undercoat layer. This is liable to cause a reduction in the properties of an undercoat layer. Furthermore, this affects the uniformity of a photosensitive layer, such as a charge generation layer or a charge transport layer, disposed on an undercoat layer and thus may be a contributing factor to the occurrence of an image failure. The thickness of a layer is adjusted by adjusting the viscosity of a coating liquid. It is thus important to inhibit variations in the viscosity of the coating liquid for an undercoat layer from the viewpoint of achieving mass production of electrophotographic photosensitive members with undercoat layers having the same thickness.
The inventors have conducted intensive studies and have found that the use of a solvent as described below inhibits the increase in the viscosity of a coating liquid for an undercoat layer with time. That is, a monohydric alcohol is used as a solvent used in a coating liquid for an undercoat layer, the coating liquid containing a polyol and a blocked isocyanate compound whose isocyanate group is blocked with the structure represented by the formula (1).
However, it was found that the simple use of the monohydric alcohol as a solvent used in the coating liquid for an undercoat layer for the purpose of stabilizing the viscosity of the coating liquid for an undercoat layer causes a black spot to form in an output image. The reason for this is presumably that the use of the monohydric alcohol as a solvent used in the coating liquid for an undercoat layer reduces the dispersibility of zinc oxide particles in the coating liquid for an undercoat layer.
To allow an undercoat layer to sufficiently provide the function of blocking charge injection, it is important that the undercoat layer should have a high uniformity of distribution of zinc oxide particles (in other words, the degree of aggregation of the zinc oxide particles be low). The reason for this is that a higher degree of aggregation of the zinc oxide particles in the undercoat layer is liable to lead to a larger number of conductive paths in the undercoat layer and that a larger number of the conductive paths in the undercoat layer is liable to cause the undercoat layer to have a reduced function of blocking charge injection.
To increase the uniformity of distribution of the zinc oxide particles in the undercoat layer, it is important that the coating liquid for an undercoat layer should have a highly uniform distribution of the zinc oxide particles.